Joshi, J., Pietarila, A., Hirzberger, J., Solanki, S. K., Aznar Cuadrado, R., & Merenda, L. (2011). Convective Nature of Sunspot Penumbral Filaments: Discovery of Downflows in the Deep Photosphere. Astrophysical Journal, 734: L18. doi:10.1088/2041-8205/734/1/L18.